Sewing machines have been provided heretofore with edge guides along which a fabric workpiece is displaced automatically or semiautomatically until the end of the seam or edge of a fabric workpiece is reached. Thereafter, it is usually necessary to effect a different type of sewing operation, e.g. stitching in place to bind off the previously formed seam, or begin the stitching of another seam angularly adjoining the first.
Such sewing machines can be used, for example, for the sewing of relatively small fabric pieces having angularly adjoining edges or corners to larger workpieces. A case in point is the sewing of a pocket to a shirt front with stitch seams along several sides of the pocket.
In such sewing machines it is known to provide a control device which generates an optical pulse as each corner is reached, i.e. when the stitching along one edge of the small workpiece reaches a corner. This pulse can trigger a setting function such as the stopping of the machine with its needle extending through the workpieces, to facilitate turning of the latter so that the next edge to be seamed engages the edge guide. The machine is then turned on again and permitted to sew a new seam along the latter edge. In other words, the control means provides means responsive to the optical sensor for performing a control function related to the stitching operation. A machine of this type is described in German published application (Auslegeschrift) DT-AS No. 1 302 988.
In this system, the optical pulse is generated by an electro-optical sensor which responds to a reflection from an indexing point or mark upon the surface of a template mounted upon the workpiece or provided upon the surface of the workpiece itself.
Naturally, the requirement for an extra piece, such as a template, is disadvantageous in that templates must be provided and stored for each of the many sizes and shapes of the workpieces to be stitched. The application of a reflecting marking point to the workpiece has the disadvantage that it requires an additional operation and hence increases the labor costs of the sewing operation.